496 research outputs found
Simulation of anisotropic wet-chemical etching using a physical model
We present a method to describe the orientation dependence of the etch rate of silicon, or any other single crystalline material, in anisotropic etching solutions by analytical functions. The parameters in these functions have a simple physical meaning. Crystals have a small number of atomically smooth faces, which etch (and grow) slowly as a consequence of the removal (or addition) of atoms by rows and layers. However, smooth faces have a roughening transition (well known in statistical physics); at increasing temperature they become rougher, and accordingly the etch and growth rates increase. Consequently, the basic physical parameters of our functions are the roughness of the smooth faces and the velocity of steps on these faces. This small set of parameters describes the etch rate in the two-dimensional space of orientations (on the unit sphere). We have applied our method to the practical case of etch rate functions for silicon crystals in KOH solutions. The maximum deviation between experimental data and simulation using only nine physically meaningful parameters is less than 5% of the maximum etch rate. This method, which in this study is used to describe anisotropic etching of silicon, can easily be adjusted to describe the growth or etching process of any crysta
Bulk screening in core level photoemission from Mott-Hubbard and Charge-Transfer systems
We report bulk-sensitive hard X-ray ( = 5.95 KeV) core level
photoemission spectroscopy (PES) of single crystal VCrO
and the high- cuprate BiSrCaCuO (Bi2212).
VCrO exhibits low binding energy "satellites" to the V
"main lines" in the metallic phase, which are suppressed in the
antiferromagnetic insulator phase. In contrast, the Cu spectra of Bi2212
do not show temperature dependent features, but a comparison with soft X-ray
PES indicates a large increase in the "satellites" or weight
in the bulk. Cluster model calculations, including full multiplet structure and
a screening channel derived from the coherent band at the Fermi energy, give
very satisfactory agreement with experiments
Scaling Propensity of Water
The commonly used Saturation Index calculated at 10°C (SI10) is not suitable for practical situations. New parameters have been developed for simple and rapid analysis of calcium carbonate precipitation (scaling) phenomena which occur during the heating of drinking water: TPCC90 (Theoretically Precipitable Calcium Carbonate at 90°C) SI90 (Saturation Index at 90°C) NI (Nucleation Index) PPCC (Practically Precipitable Calcium Carbonate) Both TPCC90 and SI90 describe the hydrodynamic driven force for the scaling reaction. The nucleation index indicates at what rate calcium carbonate precipitation is accelerated by nuclei present in the water. Finally, the PPCC is a measurement under practical conditions, determining the rate of scaling. The suitability of these parameters for predicting scaling was assessed and detailed results are presented in this publication
Transfer of Spectral Weight in Spectroscopies of Correlated Electron Systems
We study the transfer of spectral weight in the photoemission and optical
spectra of strongly correlated electron systems. Within the LISA, that becomes
exact in the limit of large lattice coordination, we consider and compare two
models of correlated electrons, the Hubbard model and the periodic Anderson
model. The results are discussed in regard of recent experiments. In the
Hubbard model, we predict an anomalous enhancement optical spectral weight as a
function of temperature in the correlated metallic state which is in
qualitative agreement with optical measurements in . We argue that
anomalies observed in the spectroscopy of the metal are connected to the
proximity to a crossover region in the phase diagram of the model. In the
insulating phase, we obtain an excellent agreement with the experimental data
and present a detailed discussion on the role of magnetic frustration by
studying the resolved single particle spectra. The results for the periodic
Anderson model are discussed in connection to recent experimental data of the
Kondo insulators and . The model can successfully explain
the different energy scales that are associated to the thermal filling of the
optical gap, which we also relate to corresponding changes in the density of
states. The temperature dependence of the optical sum rule is obtained and its
relevance for the interpretation of the experimental data discussed. Finally,
we argue that the large scattering rate measured in Kondo insulators cannot be
described by the periodic Anderson model.Comment: 19 pages + 29 figures. Submitted to PR
The Herschel Comprehensive (U)LIRG Emission Survey (HerCULES): CO Ladders, fine structure lines, and neutral gas cooling
(Ultra) Luminous Infrared Galaxies ((U)LIRGs) are objects characterized by
their extreme infrared (8-1000 m) luminosities (L and L). The Herschel Comprehensive ULIRG
Emission Survey (HerCULES; PI van der Werf) presents a representative
flux-limited sample of 29 (U)LIRGs that spans the full luminosity range of
these objects (10). With the \emph{Herschel
Space Observatory}, we observe [CII] 157 m, [OI] 63 m, and [OI] 145
m line emission with PACS, CO J=4-3 through J=13-12, [CI] 370 m, and
[CI] 609 m with SPIRE, and low-J CO transitions with ground-based
telescopes. The CO ladders of the sample are separated into three classes based
on their excitation level. In 13 of the galaxies, the [OI] 63 m emission
line is self absorbed. Comparing the CO excitation to the IRAS 60/100 m
ratio and to far infrared luminosity, we find that the CO excitation is more
correlated to the far infrared colors. We present cooling budgets for the
galaxies and find fine-structure line flux deficits in the [CII], [SiII], [OI],
and [CI] lines in the objects with the highest far IR fluxes, but do not
observe this for CO . In order to study the heating of the
molecular gas, we present a combination of three diagnostic quantities to help
determine the dominant heating source. Using the CO excitation, the CO J=1-0
linewidth, and the AGN contribution, we conclude that galaxies with large CO
linewidths always have high-excitation CO ladders, and often low AGN
contributions, suggesting that mechanical heating is important
Resonant Inelastic X-ray Scattering Studies of Elementary Excitations
In the past decade, Resonant Inelastic X-ray Scattering (RIXS) has made
remarkable progress as a spectroscopic technique. This is a direct result of
the availability of high-brilliance synchrotron X-ray radiation sources and of
advanced photon detection instrumentation. The technique's unique capability to
probe elementary excitations in complex materials by measuring their energy-,
momentum-, and polarization-dependence has brought RIXS to the forefront of
experimental photon science. We review both the experimental and theoretical
RIXS investigations of the past decade, focusing on those determining the
low-energy charge, spin, orbital and lattice excitations of solids. We present
the fundamentals of RIXS as an experimental method and then review the
theoretical state of affairs, its recent developments and discuss the different
(approximate) methods to compute the dynamical RIXS response. The last decade's
body of experimental RIXS data and its interpretation is surveyed, with an
emphasis on RIXS studies of correlated electron systems, especially transition
metal compounds. Finally, we discuss the promise that RIXS holds for the near
future, particularly in view of the advent of x-ray laser photon sources.Comment: Review, 67 pages, 44 figure
INTERLAYER COUPLING AND THE METAL-INSULATOR TRANSITION IN Pr-SUBSTITUTED Bi(2)Sr(2)CaCu(2)O(8+y)
Substitution of rare-earth ions for Ca in Bi2Sr2CaCu2O8+y is known to cause a
metal-insulator transition. Using resonant photoemission we study how this
chemical substitution affects the electronic structure of the material. For the
partial Cu-density of states at E_F and in the region of the valence band we
observe no significant difference between a pure superconducting sample and an
insulating sample with 60% Pr for Ca. This suggests that the states responsible
for superconductivity are predomi- nately O-states. The partial Pr-4f density
of states was extracted utilizing the Super- Koster-Kronig Pr 4d-4f resonance.
It consists of a single peak at 1.36eV binding energy. The peak shows a
strongly assymetric Doniach-Sunjic line- shape indicating the presence of a
continuum of electronic states with sharp cut off at E_F even in this
insulating sample. This finding excludes a bandgap in the insulating sample and
supports the existance of a mobility gap caused by spatial localization of the
carriers. The presence of such carriers at the Pr-site, between the CuO_2
planes shows that the electronic structure is not purely 2-dimensional but that
there is a finite interlayer coupling. The resonance enhancement of the
photoemission cross section, at the Pr-4d threshold, was studied for the Pr-4f
and for Cu-states. Both the Pr-4f and the Cu-states show a Fano-like resonance.
This resonance of Cu-states with Pr-states is another indication of coupling
between the the Pr-states and those in the CuO_2 plane. Because of the
statistical distribution of the Pr-ions this coupling leads to a non-periodic
potential for the states in the CuO_2 plane which can lead to localization and
thus to the observed metal-insulator transition.Comment: Gziped uuencoded postscript file including 7 figures Scheduled for
publication in Physical Review B, May 1, 1995
Developing win-win solutions for virtual placements in informatics: The VALS case
The placements and internships are one of the main paths to get professional background and some skills for students, especially in areas like informatics and computer sciences. The European-funded VALS project tries to promote the virtual placements and establish a new initiative in virtual placements called Semester of Code. This initiative binds higher education institutions, students, companies, foundations and Open Source projects in order to create virtual placements and solve needs that they have in relation with those placements. This paper introduces some projects about virtual placements that other institutions and companies perform, also the paper describes the needs, opinions and considerations about the virtual placements for each stakeholder involved in the placements, to finally explain the design decisions and actions behind the Semester of Code, and how they are intended to get better virtual placements and successful results
The influence of C3 and C4 vegetation on soil organic matter dynamics in contrasting semi-natural tropical ecosystems
This is a freely-available open access publication. Please cite the published version which is available via the DOI link in this record.Variations in the carbon isotopic composition of soil organic matter (SOM) in bulk and fractionated samples were used to assess the influence of C3 and C4 vegetation on SOM dynamics in semi-natural tropical ecosystems sampled along a precipitation gradient in West Africa. Differential patterns in SOM dynamics in C3/C4 mixed ecosystems occurred at various spatial scales. Relative changes in C/ N ratios between two contrasting SOM fractions were used to evaluate potential site-scale differences in SOM dynamics between C3- and C4-dominated locations. These differences were strongly controlled by soil texture across the precipitation gradient, with a function driven by bulk δ 13C and sand content explaining 0.63 of the observed variability. The variation of δ 13C with soil depth indicated a greater accumulation of C3-derived carbon with increasing precipitation, with this trend also being strongly dependant on soil characteristics. The influence of vegetation thickening on SOM dynamics was also assessed in two adjacent, but structurally contrasting, transitional ecosystems occurring on comparable soils to minimise the confounding effects posed by climatic and edaphic factors. Radiocarbon analyses of sand-size aggregates yielded relatively short mean residence times (τ ) even in deep soil layers, while the most stable SOM fraction associated with silt and clay exhibited shorter τ in the savanna woodland than in the neighbouring forest stand. These results, together with the vertical variation observed in δ 13C values, strongly suggest that both ecosystems are undergoing a rapid transition towards denser closed canopy formations. However, vegetation thickening varied in intensity at each site and exerted contrasting effects on SOM dynamics. This study shows that the interdependence between biotic and abiotic factors ultimately determine whether SOM dynamics of Published by Copernicus Publications on behalf of the European Geosciences Union. 5042 G. Saiz et al.: Influence of C3/C4 on SOM in tropical biomes C3- and C4-derived vegetation are at variance in ecosystems where both vegetation types coexist. The results highlight the far-reaching implications that vegetation thickening may have for the stability of deep SOM.UK National Environment Research CouncilAustralian Institute of Nuclear Science and Engineering (AINSE Ltd
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